1. Field of the Invention
This invention relates to a method of manufacturing electronic components from a leadframe. It more particularly relates to a method of producing electronic components of the type wherein each semiconductor chip is sandwiched between a pair of leads for bonding and encapsulated hermetically in a molded resinous body. The invention also relates to a leadframe for advantageously carrying out the method.
2. Description of the Prior Art
As is well known, elongated or continuous leadframes are used for quantity production of electronic components, such as diodes, transistors, and so on. Various attempts have heretofore been made to improve the productivity of such a manufacturing process and to simplify the process in addition to enhancing the quality of the products.
Two typical prior art methods of manufacturing electronic components are now described with reference to FIGS. 33 to 36 of the accompanying drawings for the convenience of explanation.
In one typical prior art method shown in FIGS. 33 to 35, a pair of parallel leadframes 11', 12' respectively formed with aligned leads 2', 3' at a constant pitch P are transferred longitudinally thereof. During such transfer, semiconductor chips 1' are bonded to the respective leads 2' of one leadframe 11', as shown in FIG. 34. Subsequently, the pair of leadframes 11', 12' are brought closer to each other, so that the leads 3' of the other leadframe 12' is overlapped on the chips 1' for bonding thereto, as shown in FIG. 35. Finally, each chip 1' together with the associated leads 2', 3' is encapsulated in a molded resin body 4' which is cut off from the respective leadframes 11', 12' to provide an individual electronic component.
However, the above prior art method is disadvantageous in that two separate leadframes 11', 12' are required for production of electronic components, thereby posing handling inconvenience (e.g. requiring two separate transfer guides, and etc.) in addition to involving material waste. Further, because of the separate nature of the leadframes 11', 12', the respective leads 2', 3', which are kept free as projecting, are likely to come into contact with external objects particularly when the leadframes 11', 12' are separately handled. As a result, the leads may be subjected to unacceptable deformation during the manufacturing process, consequently resulting in a higher chance of quality deterioration.
According to another prior art method shown in FIG. 36 of the accompanying drawings and disclosed for example in Japanese Patent Application Laid-open No. 62-35549 (Laid-open: Feb. 16, 1987; Application No.: 60-176036; Filed: Aug. 8, 1985; Applicant: Mitsubishi Electric Corp.), an integral leadframe 10" is used which includes a pair of longitudinal bands 11", 12" connected together by transverse sectioning bars 13" spaced longitudinally of the leadframe at a constant pitch P. One longitudinal band 11" is formed with a group of first leads (longer leads) 2" which are spaced longitudinally of the leadframe and extend toward the other longitudinal band 12", whereas the other band 12" is formed with a group of second leads (shorter leads) 3" which are also spaced longitudinally of the leadframe and extend toward the one longitudinal band 11" in staggered relation to the group of first leads 2".
In manufacture, the integral leadframe 10" is transferred longitudinally thereof. During such transfer, a semiconductor chip 1" is bonded to each second lead 3". Subsequently, a corresponding first lead 2" is bent along a bending line C", so that the bent portion is overlapped on the chip 1". Finally, the chip together with the associated leads is hermetically molded in a resin body 4", and cut off from the leadframe to provide an individual electronic component.
Compared with the method shown in FIGS. 33 to 35, the method of FIG. 36 is advantageous in that electronic components can be produced from the single leadframe 10" which enables easier handling. Further, all of the leads 2", 3" are always located in the same plane of the leadframe throughout various stages of the manufacturing process. Thus, the longitudinal space between the two longitudinal bands 11", 12" is an area always protected by these bands, so that the leads 2", 3" arranged in this protected area are less likely to come into contact with external objects. Therefore, it is possible to avoid unexpected deformation of the leads, thereby improving the quality of the final products.
However, the prior art method of FIG. 36 is also disadvantageous in various points. First, the bent portion of each longer lead 2" causes a weight increase of the individual electronic component in addition to involving material waste. Secondly, each pair of leads 2", 3" are spaced longitudinally of the leadframe C before and after the molding operation, so that the resulting product must necessarily have an increased width S which also causes a weight increase. In the third place, this width increase also results in decrease in number of the products obtainable per unit length of the leadframe 10', thus causing material waste.
More importantly, the bending of the first leads 2" requires a relatively large force and therefore necessitates a slowdown in the transfer speed of the leadframe. Combined with the width increase of the individual products (therefore a length increase of the leadframe), such a slowdown greatly reduces the productivity of the manufacturing process, thus resulting in a cost increase.
In either of the prior art methods described above, the molded resinous body 4' (or 4") is formed by using a pair of molds (an upper mold and a lower mold) which, when closed, form a cavity for receiving injected fluid thermosetting resin. Obviously, one or both of the molds must be formed with grooves through which the first and second leads 2', 3' (or 2", 3") project out when sealingly closing the molds.
Indeed, it is very difficult to fit the projecting leads into the grooves of the molds in a well-sealed condition, and certain clearances are inevitably formed at the grooves even after closing the molds. Thus, when fluid resin is injected into the mold cavity, an small amount of resin enters into the grooves to form flashes.
The resinous flashes thus formed may come off and get into the mechanical and/or electrical systems during the subsequent process steps, consequently leading to operation troubles. Thus, it is necessary to provide a separate process unit for removing the flashes immediately after formation of the molded body, thereby resulting in production cost increase.